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LT8645S/LT8646S
1Rev. B
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TYPICAL APPLICATION
FEATURES DESCRIPTION
65V, 8A Synchronous Step-Down Silent Switcher 2
with 2.5µA Quiescent Current
The LT®8645S/LT8646S synchronous step-down regulator features second generation Silent Switcher architecture designed to minimize EMI emissions while delivering high efficiency at high switching frequencies. This includes the integration of bypass capacitors to optimize all the fast current loops inside and make it easy to achieve advertised EMI performance by eliminating layout sensitivity.
The fast, clean, low-overshoot switching edges enable high efficiency operation even at high switching frequen-cies, leading to a small overall solution size. Peak current mode control with a 40ns minimum on-time allows high step-down ratios even at high switching frequencies. The LT8646S has external compensation to enable current sharing and fast transient response at high switching frequencies.
Burst Mode® operation enables ultralow standby current consumption, pulse-skipping mode allows full switching frequency at lower output loads, or spread spectrum operation can further reduce EMI emissions.
INTERNAL CAPS VC COMP 150°C GRADE
LT8645S Yes Internal No
LT8646S Yes External No
LT8645S-2* No Internal Yes
*See LT8645S-2 Data Sheet
5V 8A Step-Down Converter
APPLICATIONS
n Silent Switcher®2 Architecture n Ultralow EMI Emissions on Any PCB n Eliminates PCB Layout Sensitivity n Internal Bypass Capacitors Reduce Radiated EMI n Optional Spread Spectrum Modulation
n High Efficiency at High Frequency n Up to 95% Efficiency at 1MHz, 12VIN to 5VOUT n Up to 94% Efficiency at 2MHz, 12VIN to 5VOUT
n Wide Input Voltage Range: 3.4V to 65V n Ultralow Quiescent Current Burst Mode Operation
n 2.5μA IQ Regulating 12VIN to 3.3VOUT (LT8645S) n Output Ripple < 10mVP-P
n External Compensation: Fast Transient Response and Current Sharing (LT8646S)
n Fast Minimum Switch On-Time: 40ns n Low Dropout Under All Conditions: 60mV at 1A n Adjustable and Synchronizable: 200kHz to 2.2MHz n Peak Current Mode Operation n Output Soft-Start and Tracking n Small 32-Lead 6mm × 4mm LQFN Package n AEC-Q100 Qualified for Automotive Applications
n Automotive and Industrial Supplies n General Purpose Step-Down n GSM Power Supplies All registered trademarks and trademarks are the property of their respective owners. Protected
by U.S. patents, including 8823345.
12VIN to 5VOUT Efficiency
LT8645S
8645S TA01a
SW
BIAS
FB
VIN5.5V TO 65V
GND
RT
VIN
EN/UV
VOUT5V8A
100µF
2.2pF
4.7µF
41.2k
1M
243k
2.2µH
fSW = 1MHz
EFFICIENCY
POWER LOSS
1MHz, L = 2.2µH2MHz, L = 1µH
LOAD CURRENT (A)1 2 3 4 5 6 7 8
60
65
70
75
80
85
90
95
100
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
EFFI
CIEN
CY (%
)
POWER LOSS (W
)
8645S TA01b
LT8645S/LT8646S
2Rev. B
For more information www.analog.com
PIN CONFIGURATION
ABSOLUTE MAXIMUM RATINGSVIN, EN/UV ................................................................65VPG .............................................................................42VBIAS ..........................................................................25VFB, TR/SS . .................................................................4VSYNC/MODE Voltage . ................................................6V
(Note 1)
LT8645S LT8646S
LQFN PACKAGE32-LEAD (6mm × 4mm × 0.94mm)
JEDEC BOARD: θJA = 30°C/W, ΨJT = 0.6°C/W, θJCTOP = 28.5°C/W, θJC(PAD) = 4.4°C/W (NOTE 3)
DEMO BOARD: θJA = 17°C/WEXPOSED PADS (PINS 33-38) ARE GND, SHOULD BE SOLDERED TO PCB
TOP VIEW
11 12 13 14 15 16
32 31 30 29 28 27
21
17
18
19
20
26
22
23
24
25
6
10
9
8
7
1
5
4
3
2
VIN
GND
GND
GND
NC
BIAS
VIN
VIN
NC
INTVCC
VIN
GND
GND
GND
NC
RT
VIN
VIN
NC
EN/UV33
GND34
GND
35GND
36GND
37GND
38GND
BST
SW SW SW SW SW
FB PG GND
TR/S
S
SYNC
/MOD
E
CLKO
UT
LQFN PACKAGE32-LEAD (6mm × 4mm × 0.94mm)
JEDEC BOARD: θJA = 30°C/W, ΨJT = 0.6°C/W, θJCTOP = 28.5°C/W, θJC(PAD) = 4.4°C/W (NOTE 3)
DEMO BOARD: θJA = 17°C/WEXPOSED PADS (PINS 33-38) ARE GND, SHOULD BE SOLDERED TO PCB
TOP VIEW
11 12 13 14 15 16
32 31 30 29 28 27
21
17
18
19
20
26
22
23
24
25
6
10
9
8
7
1
5
4
3
2
VIN
GND
GND
GND
NC
BIAS
VIN
VIN
NC
INTVCC
VIN
GND
GND
GND
NC
RT
VIN
VIN
NC
EN/UV33
GND34
GND
35GND
36GND
37GND
38GND
BST
SW SW SW SW SW
FB PG V C TR/S
S
SYNC
/MOD
E
CLKO
UT
Operating Junction Temperature Range (Note 2) LT8645SE/LT8646SE ......................... –40°C to 125°C LT8645SI/LT8646SI........................... –40°C to 125°CStorage Temperature Range .................. –65°C to 150°CMaximum Reflow (Package Body) Temperature ... 260°C
LT8645S/LT8646S
3Rev. B
For more information www.analog.com
ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C.
ORDER INFORMATION
PARAMETER CONDITIONS MIN TYP MAX UNITS
Minimum Input Voltage l 3.0 3.4 V
VIN Quiescent Current in Shutdown VEN/UV = 0V
l
0.9 0.9
3 10
µA µA
LT8645S VIN Quiescent Current in Sleep (Internal Compensation)
VEN/UV = 2V, VFB > 0.97V, VSYNC = 0V
l
1.7 1.7
4 10
µA µA
LT8646S VIN Quiescent Current in Sleep (External Compensation)
VEN/UV = 2V, VFB > 0.97V, VSYNC = 0V, VBIAS = 0V
l
230 230
290 340
µA µA
VEN/UV = 2V, VFB > 0.97V, VSYNC = 0V, VBIAS = 5V 16 25 µA
LT8646S BIAS Quiescent Current in Sleep VEN/UV = 2V, VFB > 0.97V, VSYNC = 0V, VBIAS = 5V 200 260 µA
LT8645S VIN Quiescent Current when Active VEN/UV = 2V, VFB > 0.97V, VSYNC = 2V, RT = 60.4k, VBIAS = 0V 0.4 0.6 mA
LT8646S VIN Quiescent Current when Active VEN/UV = 2V, VFB > 0.97V, VSYNC = 2V, RT = 60.4k, VBIAS = 0V 0.6 0.8 mA
LT8645S VIN Current in Regulation VOUT = 0.97V, VIN = 6V, ILOAD = 100µA, VSYNC = 0V VOUT = 0.97V, VIN = 6V, ILOAD = 1mA, VSYNC = 0V
l
l
17 200
60 400
µA µA
Feedback Reference Voltage VIN = 6V VIN = 6V
l
0.964 0.958
0.970 0.970
0.976 0.982
V V
Feedback Voltage Line Regulation VIN = 4.0V to 42V l 0.004 0.025 %/V
Feedback Pin Input Current VFB = 1V –20 20 nA
LT8646S Error Amp Transconductance VC = 1.25V 1.7 mS
LT8646S Error Amp Gain 350 V/V
LT8646S VC Source Current VFB = 0.77V, VC = 1.25V 350 µA
LT8646S VC Sink Current VFB = 1.17V, VC = 1.25V 350 µA
PART NUMBER PART MARKING* FINISH CODE PAD FINISH PACKAGE TYPE***MSL
RATING TEMPERATURE RANGELT8645SEV#PBF
8645SV
e4 Au (RoHS) LQFN (Laminate Package with QFN Footprint) 3 –40°C to 125°C
LT8645SIV#PBF
LT8646SEV#PBF8646SV
LT8646SIV#PBF
AUTOMOTIVE PRODUCTS**LT8645SEV#WPBF
8645SV
e4 Au (RoHS) LQFN (Laminate Package with QFN Footprint) 3 –40°C to 125°C
LT8645SIV#WPBF
LT8646SEV#WPBF8646SV
LT8646SIV#WPBF
• Contact the factory for parts specified with wider operating temperature ranges. *Pad or ball finish code is per IPC/JEDEC J-STD-609.
• Recommended LGA and BGA PCB Assembly and Manufacturing Procedures
• LGA and BGA Package and Tray Drawings
Parts ending with PBF are RoHS and WEEE compliant. ***The LT8645S/LT8646S package has the same footprint as a standard 6mm × 4mm QFN Package.
**Versions of this part are available with controlled manufacturing to support the quality and reliability requirements of automotive applications. These models are designated with a #W suffix. Only the automotive grade products shown are available for use in automotive applications. Contact your local Analog Devices account representative for specific product ordering information and to obtain the specific Automotive Reliability reports for these models.
LT8645S/LT8646S
4Rev. B
For more information www.analog.com
PARAMETER CONDITIONS MIN TYP MAX UNITS
LT8646S VC Pin to Switch Current Gain 8 A/V
LT8646S VC Clamp Voltage 2.6 V
BIAS Pin Current Consumption VBIAS = 3.3V, fSW = 2MHz 22 mA
Minimum On-Time ILOAD = 2A, SYNC = 0V ILOAD = 2A, SYNC = 2V
l
l
40 35
65 60
ns ns
Minimum Off-Time 80 110 ns
Oscillator Frequency RT = 221k RT = 60.4k RT = 18.2k
l
l
l
180 665 1.8
210 700 1.95
240 735 2.1
kHz kHz
MHz
Top Power NMOS On-Resistance ISW = 1A 36 mΩ
Top Power NMOS Current Limit l 10.5 14 17.5 A
Bottom Power NMOS On-Resistance VINTVCC = 3.4V, ISW = 1A 25 mΩ
Bottom Power NMOS Current Limit VINTVCC = 3.4V 8.5 11 13.5 A
SW Leakage Current VIN = 42V, VSW = 0V, 42V –1.5 1.5 µA
EN/UV Pin Threshold EN/UV Rising l 0.95 1.01 1.07 V
EN/UV Pin Hysteresis 45 mV
EN/UV Pin Current VEN/UV = 2V –20 20 nA
PG Upper Threshold Offset from VFB VFB Falling l 5 7.5 10 %
PG Lower Threshold Offset from VFB VFB Rising l –10.5 –8 –5.5 %
PG Hysteresis 0.4 %
PG Leakage VPG = 3.3V –40 40 nA
PG Pull-Down Resistance VPG = 0.1V l 750 2000 Ω
SYNC/MODE Threshold SYNC/MODE DC and Clock Low Level Voltage SYNC/MODE Clock High Level Voltage SYNC/MODE DC High Level Voltage
l
l
l
0.7
2.2
0.9 1.2
2.55
1.4 2.9
V V V
Spread Spectrum Modulation Frequency Range
RT = 60.4k, VSYNC = 3.3V 24 %
Spread Spectrum Modulation Frequency VSYNC = 3.3V 2.5 kHz
TR/SS Source Current l 1.2 1.9 2.6 µA
TR/SS Pull-Down Resistance Fault Condition, TR/SS = 0.1V 220 Ω
Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime.Note 2: The LT8645SE/LT8646SE is guaranteed to meet performance specifications from 0°C to 125°C junction temperature. Specifications over the –40°C to 125°C operating junction temperature range are assured by design, characterization, and correlation with statistical process controls. The LT8645SI/LT8646SI is guaranteed over the full –40°C to 125°C operating junction temperature range. The junction temperature (TJ, in °C) is calculated from the ambient temperature (TA in °C) and power dissipation (PD, in Watts) according to the formula: TJ = TA + (PD • θJA)where θJA (in °C/W) is the package thermal impedance.
Note 3: θ values determined per JEDEC 51-7, 51-12. See Applications Information Section for information on improving the thermal resistance and for actual temperature measurements of a demo board in typical operating conditions.Note 4: This IC includes overtemperature protection that is intended to protect the device during overload conditions. Junction temperature will exceed 150°C when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature will reduce lifetime.
ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C.
LT8645S/LT8646S
5Rev. B
For more information www.analog.com
TYPICAL PERFORMANCE CHARACTERISTICS
12VIN to 5VOUT Efficiency vs Frequency
12VIN to 3.3VOUT Efficiency vs Frequency Efficiency at 5VOUT
Efficiency at 3.3VOUT
LT8645S Low Load Efficiency at 5VOUT
LT8645S Low Load Efficiency at 3.3VOUT Efficiency vs Frequency
LT8646S Low Load Efficiency at 5VOUT
LT8646S Low Load Efficiency at 3.3VOUT
EFFICIENCY
POWER LOSSL = XEL6060
500kHz, L = 2.7µH1MHz, L = 2.2µH2MHz, L = 1µH
LOAD CURRENT (A)1 2 3 4 5 6 7 8
60
65
70
75
80
85
90
95
100
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
EFFI
CIEN
CY (%
)
POWER LOSS (W
)
8645S G01
EFFICIENCY
POWER LOSSL = XEL6060
500kHz, L = 2.7µH1MHz, L = 1.5µH2MHz, L = 0.82µH
LOAD CURRENT (A)1 2 3 4 5 6 7 8
60
65
70
75
80
85
90
95
100
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
EFFI
CIEN
CY (%
)POW
ER LOSS (W)
8645S G02
fSW = 500kHzL = XEL6060, 2.7µH
EFFICIENCY
POWER LOSS
VIN = 12VVIN = 24VVIN = 36VVIN = 48V
LOAD CURRENT (A)0 1 2 3 4 5 6 7 8
50
55
60
65
70
75
80
85
90
95
100
0
0.4
0.8
1.2
1.6
2.0
2.4
2.8
3.2
3.6
4.0
EFFI
CIEN
CY (%
)
POWER LOSS (W
)
8645S G03
fSW = 500kHzL = XEL6060, 2.7µH
EFFICIENCY
POWER LOSS
VIN = 12VVIN = 24VVIN = 36VVIN = 48V
LOAD CURRENT (A)0 1 2 3 4 5 6 7 8
50
55
60
65
70
75
80
85
90
95
100
0
0.4
0.8
1.2
1.6
2.0
2.4
2.8
3.2
3.6
4.0
EFFI
CIEN
CY (%
)
POWER LOSS (W
)
8645S G04
VOUT = 3.3VILOAD = 2AL = XEL6060, 4.7µH
VIN = 12V
VIN = 24V
SWITCHING FREQUENCY (MHz)0.4 0.7 1 1.3 1.6 1.9 2.2
80
82
84
86
88
90
92
94
96
EFFI
CIEN
CY (%
)
8645S G09
fSW = 500kHzL = WE–LHMI7050, 4.7µH
VIN = 12VVIN = 24VVIN = 36VVIN = 48V
LOAD CURRENT (mA)0.01 0.1 1 10 100 1000
20
30
40
50
60
70
80
90
100
EFFI
CIEN
CY (%
)
8645S G05
fSW = 500kHzL = WE–LHMI7050, 4.7µH
VIN = 12VVIN = 24VVIN = 36VVIN = 48V
LOAD CURRENT (mA)0.1 1 10 100 1000
10
20
30
40
50
60
70
80
90
100
EFFI
CIEN
CY (%
)
OUT
8645S G06
fSW = 500kHzL = WE–LHMI7050, 4.7µH
VIN = 12VVIN = 24VVIN = 36VVIN = 48V
LOAD CURRENT (mA)0.01 0.1 1 10 100 1000
20
30
40
50
60
70
80
90
100
EFFI
CIEN
CY (%
)
8645S G07
fSW = 500kHzL = WE–LHMI7050, 4.7µH
VIN = 12VVIN = 24VVIN = 36VVIN = 48V
LOAD CURRENT (mA)0.1 1 10 100 1000
10
20
30
40
50
60
70
80
90
100
EFFI
CIEN
CY (%
)
LT8646S Low Load Efficiency at 3.3VOUT
8645S G08
LT8645S/LT8646S
6Rev. B
For more information www.analog.com
TYPICAL PERFORMANCE CHARACTERISTICS
EN Pin ThresholdsReference Voltage
LT8645S Load Regulation LT8645S Line Regulation
LT8645S No-Load Supply Current
Burst Mode Operation Efficiency vs Inductor Value (LT8645S)
LT8646S Load Regulation
LT8646S Line Regulation LT8646S No-Load Supply Current
VOUT = 5VILOAD = 10mAL = WE–LHMI7050
VIN = 12V
VIN = 24V
INDUCTOR VALUE (µH)1 2 3 4 5 6 7 8 9 10
65
70
75
80
85
90
95
100
EFFI
CIEN
CY (%
)
8645S G10TEMPERATURE (°C)
–50 –25 0 25 50 75 100 125961
963
965
967
969
971
973
975
977
979
REFE
RENC
E VO
LTAG
E (m
V)
8645S G11
EN RISING
EN FALLING
TEMPERATURE (°C)–50 –25 0 25 50 75 100 125
0.95
0.96
0.97
0.98
0.99
1.00
1.01
1.02
1.03
EN T
HRES
HOLD
(V)
8645S G12
LOAD CURRENT (A)0 1 2 3 4 5 6 7 8
–0.10
–0.05
0
0.05
0.10
0.15
0.20
CHAN
GE IN
VOU
T (%
)
8645S G13
VOUT = 5VVIN = 12VVSYNC = 0V
VOUT = 5VVIN = 12VVSYNC = 0V
LOAD CURRENT (A)0 1 2 3 4 5 6 7 8
–0.50
–0.40
–0.30
–0.20
–0.10
0.00
0.10
0.20
0.30
0.40
CHAN
GE IN
VOU
T (%
)
8645S G14
VOUT = 5VILOAD = 2A
INPUT VOLTAGE (V)5 15 25 35 45 55 65
–0.10
–0.05
0.00
0.05
0.10
0.15
0.20
CHAN
GE IN
VOU
T (%
)
8645S G15
VOUT = 5VILOAD = 2A
INPUT VOLTAGE (V)5 15 25 35 45 55 65
–0.10
–0.05
0
0.05
0.10
0.15
0.20
0.25
0.30
CHAN
GE IN
VOU
T (%
)
LT8646S Line Regulation
8645S G16
VOUT = 5VL = 4.7µHIN REGULATION
INPUT VOLTAGE (V)0 10 20 30 40 50 60
25
50
75
100
125
150
175
200
225
INPU
T CU
RREN
T (µ
A)
8645S G18
VOUT = 3.3VL = 4.7µHIN-REGULATION
INPUT VOLTAGE (V)0 10 20 30 40 50 60
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
INPU
T CU
RREN
T (µ
A)
8645S G17
LT8645S/LT8646S
7Rev. B
For more information www.analog.com
Top FET Current Limit vs Duty Cycle Top FET Current Limit Switch Drop vs Temperature
Switch Drop vs Switch Current
TYPICAL PERFORMANCE CHARACTERISTICS
Dropout Voltage Minimum On-Time
Switching Frequency Burst FrequencyMinimum Load to Full Frequency (Pulse-Skipping Mode)
DUTY CYCLE0.1 0.3 0.5 0.7 0.9
11.0
11.5
12.0
12.5
13.0
13.5
14.0
14.5
15.0
CURR
ENT
LIM
IT (A
)
8645S G19
5% DC
TEMPERATURE (°C)–50 –25 0 25 50 75 100 125
12
13
14
15
16
CURR
ENT
LIM
IT (A
)
8645S G20
TOP SWITCH
BOTTOM SWITCH
SWITCH CURRENT = 1A
TEMPERATURE (°C)–50 –25 0 25 50 75 100 125
0
20
40
60
80
100
SWIT
CH D
ROP
(mV)
8645S G21
TOP SWITCH
BOTTOM SWITCH
SWITCH CURRENT (A)0 1 2 3 4 5 6 7
0
50
100
150
200
250
300
350
SWIT
CH D
ROP
(mV)
8645S G22
VIN = 5VVOUT SET TO REGULATE AT 5VL = XEL6060, 1µH
LOAD CURRENT (A)0 1 2 3 4 5 6 7
0
50
100
150
200
250
300
350
400
DROP
OUT
VOLT
AGE
(mV)
8645S G23
ILOAD = 3AVOUT = 0.97VfSW = 2.2MHz
BURST MODE OPERATIONPULSE–SKIPPING MODE
TEMPERATURE (°C)–50 –25 0 25 50 75 100 125
25
30
35
40
45
50
MIN
IMUM
ON-
TIM
E (n
s)
8645S G24
RT = 60.4k
TEMPERATURE (°C)–50 –25 0 25 50 75 100 125
660
670
680
690
700
710
720
730
740
SWIT
CHIN
G FR
EQUE
NCY
(kHz
)
8645S G25
FRONT PAGE APPLICATIONVIN = 12VVOUT = 5V
LOAD CURRENT (mA)0 100 200 300 400 500 600
0
200
400
600
800
1000
1200
SWIT
CHIN
G FR
EQUE
NCY
(kHz
)
8645S G26
FRONT PAGE APPLICATIONVOUT = 5VfSW = 1MHzVSYNC = Float
INPUT VOLTAGE (V)5 15 25 35 45 55 65
0
50
100
150
200
250
300
350
400
LOAD
CUR
RENT
(mA)
8645S G27
LT8645S/LT8646S
8Rev. B
For more information www.analog.com
TYPICAL PERFORMANCE CHARACTERISTICS
Soft-Start Current
PG High Thresholds PG Low Thresholds
RT Programmed Switching Frequency Minimum Input Voltage Bias Pin Current
LT8645S Soft-Start Tracking LT8646S Soft-Start Tracking
LT8646S Error Amp Output Current
TR/SS VOLTAGE (V)0
FB V
OLTA
GE (V
) 0.8
1.0
1.2
0.6 1.0
8645S G28
0.6
0.4
0.2 0.4 0.8 1.2 1.4
0.2
0
TR/SS VOLTAGE (V)0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
0
0.2
0.4
0.6
0.8
1.0
1.2
FB V
OLTA
GE (V
)
LT8646S Soft-Start Tracking
8645S G29
VSS = 0.5V
TEMPERATURE (°C)–50 –25 0 25 50 75 100 125
1.4
1.5
1.6
1.7
1.8
1.9
2.0
2.1
2.2
TR/S
S PI
N CU
RREN
T (µ
A)
8645S G30
VC = 1.25V
FB PIN ERROR VOLTAGE (mV)–200 –100 0 100 200
–500
–375
–250
–125
0
125
250
375
500
V CC
PIN
CUR
RENT
(µA)
LT8646S Error Amp Output Current
8645S G31
FB RISING
FB FALLING
TEMPERATURE (°C)–50 –25 0 25 50 75 100 125
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
PG T
HRES
HOLD
OFF
SET
FROM
VRE
F (%
)
8645S G32
FB RISING
FB FALLING
TEMPERATURE (°C)–50 –25 0 25 50 75 100 125
–10.0
–9.5
–9.0
–8.5
–8.0
–7.5
–7.0
–6.5
–6.0
PG T
HRES
HOLD
OFF
SET
FROM
VRE
F (%
)
8645S G33
SWITCHING FREQUENCY (MHz)0.2
RT P
IN R
ESIS
TOR
(kΩ
)
150
200
250
1.8
8645S G34
100
50
125
175
225
75
25
00.6 1 1.4 2.2
TEMPERATURE (°C)–50 –25 0 25 50 75 100 125
2.4
2.6
2.8
3.0
3.2
3.4
3.6
INPU
T VO
LTAG
E (V
)
8645S G35
VBIAS = 5VVOUT = 5VfSW = 1MHz
INPUT VOLTAGE (V)5 15 25 35 45 55 65
10
11
12
13
14
15
BIAS
PIN
CUR
RENT
(mA)
8645S G36
LT8645S/LT8646S
9Rev. B
For more information www.analog.com
TYPICAL PERFORMANCE CHARACTERISTICS
Bias Pin Current Case Temperature Rise
Switch Rising EdgeSwitching Waveforms, Full Frequency Continuous Operation
Switching Waveforms, Burst Mode Operation Switching Waveforms
VBIAS = 5VVOUT = 5VVIN = 12VILOAD = 1A
SWITCHING FREQUENCY (MHz)0.2 0.6 1 1.4 1.8 2.2
0
5
10
15
20
25
30
BIAS
PIN
CUR
RENT
(mA)
8645S G37
DC2468A DEMO BOARDVIN = 12V, fSW = 500kHzVIN = 24V, fSW = 500kHzVIN = 12V, fSW = 2MHzVIN = 24V, fSW = 2MHz
LOAD CURRENT (A)0 1 2 3 4 5 6 7 8
0
10
20
30
40
50
60
70
80
CASE
TEM
PERA
TURE
RIS
E (°
C)
8645S G38
VIN = 12VILOAD = 3A
2ns/DIV
VSW2V/DIV
8645S G39
FRONT PAGE APPLICATION12VIN TO 5VOUT AT 2A
500ns/DIV
VSW5V/DIV
IL1A/DIV
8645S G40
FRONT PAGE APPLICATION12VIN TO 5VOUT AT 10mAVSYNC = 0V
10µs/DIV
VSW5V/DIV
IL500mA/DIV
8645S G41
FRONT PAGE APPLICATION48VIN TO 5VOUT AT 2A
500ns/DIV
VSW20V/DIV
IL1A/DIV
8645S G42
LT8645S/LT8646S
10Rev. B
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TYPICAL PERFORMANCE CHARACTERISTICS
Start-Up Dropout Performance Start-Up Dropout Performance
LT8646S Transient Response; 300mA (Burst Mode Operation) to 1.3A Transient
LT8645S Transient Response; Internal Compensation
LT8645S Transient Response; 300mA (Burst Mode Operation) to 1.3A Transient
LT8646S Transient Response; External CompensationInternal Compensation
2A TO 4A TRANSIENT12VIN, 5VOUT, fSW = 2MHzCOUT = 100µF, CLEAD = 4.7pF
20µs/DIV
VOUT100mV/DIV
ILOAD2A/DIV
8645S G43
2A TO 4A TRANSIENT12VIN, 5VOUT, fSW = 2MHzCC = 330pF, RC = 7.5kCOUT = 100µF, CLEAD = 4.7pF
20µs/DIV
VOUT100mV/DIV
ILOAD2A/DIV
8645S G44
to 1.3A Transient
300mA TO 1.3A TRANSIENT12VIN, 5VOUT, fSW = 2MHzCOUT = 100µF, CLEAD = 4.7pF
50µs/DIV
VOUT100mV/DIV
ILOAD1A/DIV
8645S G45
300mA TO 1.3A TRANSIENT12VIN, 5VOUT, fSW = 2MHzCC = 330pF, RC = 7.5kCOUT = 100µF, CLEAD = 4.7pF
50µs/DIV
VOUT100mV/DIV
ILOAD1A/DIV
8645S G46
VIN2V/DIV
VOUT2V/DIV
100ms/DIV2.5Ω LOAD(2A IN REGULATION)
8645S G47
VIN
VOUT
VIN2V/DIV
VOUT2V/DIV
100ms/DIV20Ω LOAD(250mA IN REGULATION)
8645S G48
VIN
VOUT
LT8645S/LT8646S
11Rev. B
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Radiated EMI Performance (CISPR25 Radiated Emission Test with Class 5 Peak Limits)
TYPICAL PERFORMANCE CHARACTERISTICS
Conducted EMI Performance
DC2468A DEMO BOARD(WITH EMI FILTER INSTALLED)14V INPUT TO 5V OUTPUT AT 4A, fSW = 2MHz
FIXED FREQUENCY MODESPREAD SPECTRUM MODE
FREQUENCY (MHz)0 3 6 9 12 15 18 21 24 27 30
–40
–30
–20
–10
0
10
20
30
40
50
60
AMPL
ITUD
E (d
BµV)
8645S G49
VERTICAL POLARIZATIONPEAK DETECTOR
CLASS 5 PEAK LIMITSPREAD SPECTRUM MODEFIXED FREQUENCY MODE
FREQUENCY (MHz)0 100 200 300 400 500 600 700 800 900 1000
–5
0
5
10
15
20
25
30
35
40
45
50
AMPL
ITUD
E (d
BµV/
m)
8645S G50
DC2468A DEMO BOARD(WITH EMI FILTER INSTALLED)14V INPUT TO 5V OUTPUT AT 4A, fSW = 2MHz
HORIZONTAL POLARIZATIONPEAK DETECTOR
CLASS 5 PEAK LIMITSPREAD SPECTRUM MODEFIXED FREQUENCY MODE
FREQUENCY (MHz)0 100 200 300 400 500 600 700 800 900 1000
–5
0
5
10
15
20
25
30
35
40
45
50
AMPL
ITUD
E (d
BµV/
m)
8645S G51
LT8645S/LT8646S
12Rev. B
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PIN FUNCTIONSBIAS (Pin 1): The internal regulator will draw current from BIAS instead of VIN when BIAS is tied to a voltage higher than 3.1V. For output voltages of 3.3V to 25V this pin should be tied to VOUT. If this pin is tied to a supply other than VOUT use a 1µF local bypass capacitor on this pin. If no supply is available, tie to GND. However, especially for high input or high frequency applications, BIAS should be tied to output or an external supply of 3.3V or above.
INTVCC (Pin 2): Internal 3.4V Regulator Bypass Pin. The internal power drivers and control circuits are powered from this voltage. INTVCC maximum output current is 25mA. Do not load the INTVCC pin with external circuitry. INTVCC current will be supplied from BIAS if BIAS > 3.1V, otherwise current will be drawn from VIN. Voltage on INTVCC will vary between 2.8V and 3.4V when BIAS is between 3.0V and 3.6V. This pin should be floated.
NC (Pins 3, 7, 20, 24): No Connect. This pin is not con-nected to internal circuitry and can be tied anywhere on the PCB, typically ground.
VIN (Pins 4, 5, 6, 21, 22, 23): The VIN pins supply cur-rent to the LT8645S/LT8646S internal circuitry and to the internal topside power switch. These pins must be tied together and be locally bypassed with a capacitor of 4.7µF or more. Be sure to place the positive terminal of the input capacitor as close as possible to the VIN pins, and the negative capacitor terminal as close as possible to the GND pins. See the Applications Information section for a sample layout.
GND (Pins 8, 9, 10, 17, 18, 19, Exposed Pad Pins 33–38): Ground. Place the negative terminal of the input capacitor as close to the GND pins as possible. See the Applications Information section for a sample layout. The exposed pads should be soldered to the PCB for good thermal performance. If necessary due to manufacturing limitations Pins 33 to 38 may be left disconnected, however thermal performance will be degraded.
BST (Pin 11): This pin is used to provide a drive voltage, higher than the input voltage, to the topside power switch. This pin should be floated.
SW (Pins 12, 13, 14, 15, 16): The SW pins are the outputs of the internal power switches. Tie these pins together and connect them to the inductor and boost capacitor. This node should be kept small on the PCB for good performance and low EMI.
EN/UV (Pin 25): The LT8645S/LT8646S is shut down when this pin is low and active when this pin is high. The hysteretic threshold voltage is 1.01V going up and 0.965V going down. Tie to VIN if the shutdown feature is not used. An external resistor divider from VIN can be used to program a VIN threshold below which the LT8645S/LT8646S will shut down.
RT (Pin 26): A resistor is tied between RT and ground to set the switching frequency.
CLKOUT (Pin 27): In pulse-skipping mode, spread spec-trum, and synchronization modes, the CLKOUT pin will provide a ~200ns wide pulse at the switch frequency. The low and high levels of the CLKOUT pin are ground and INTVCC respectively, and the drive strength of the CLKOUT pin is several hundred ohms. In Burst Mode operation, the CLKOUT pin will be low. Float this pin if the CLKOUT function is not used.
SYNC/MODE (Pin 28): This pin programs four different operating modes: 1) Burst Mode. Tie this pin to ground for Burst Mode operation at low output loads—this will result in ultralow quiescent current. 2) Pulse-skipping mode. This mode offers full frequency operation down to low output loads before pulse skipping occurs. Float this pin for pulse-skipping mode. When floating, pin leakage currents should be <1µA. 3) Spread spectrum mode. Tie this pin high to INTVCC (~3.4V) or an external supply of 3V to 4V for pulse-skipping mode with spread spectrum modulation. 4) Synchronization mode. Drive this pin with a clock source to synchronize to an external frequency. During synchronization the part will operate in pulse-skipping mode.
LT8645S/LT8646S
13Rev. B
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PIN FUNCTIONSTR/SS (Pin 29): Output Tracking and Soft-Start Pin. This pin allows user control of output voltage ramp rate during start-up. For the LT8645S, a TR/SS voltage below 0.97V forces it to regulate the FB pin to equal the TR/SS pin volt-age. When TR/SS is above 0.97V, the tracking function is disabled and the internal reference resumes control of the error amplifier. For the LT8646S, a TR/SS voltage below 1.6V forces it to regulate the FB pin to a function of the TR/SS pin voltage. See plot in the Typical Performance Characteristics section. When TR/SS is above 1.6V, the tracking function is disabled and the internal reference resumes control of the error amplifier. An internal 1.9μA pull-up current from INTVCC on this pin allows a capacitor to program output voltage slew rate. This pin is pulled to ground with an internal 200Ω MOSFET during shutdown and fault conditions; use a series resistor if driving from a low impedance output. This pin may be left floating if the tracking function is not needed.
GND (Pin 30 LT8645S Only): Ground. Connect this pin to system ground and to the ground plane. This pin is also connected to ground internally, and can be left floating on PCB to be pin compatible with the LT8646S.
VC (Pin 30, LT8646S Only): The VC pin is the output of the internal error amplifier. The voltage on this pin controls the peak switch current. Tie an RC network from this pin to ground to compensate the control loop.
PG (Pin 31): The PG pin is the open-drain output of an internal comparator. PG remains low until the FB pin is within ±8% of the final regulation voltage, and there are no fault conditions. PG is pulled low when EN/UV is below 1V, INTVCC has fallen too low, VIN is too low, or thermal shutdown. PG is valid when VIN is above 3.4V.
FB (Pin 32): The LT8645S/LT8646S regulates the FB pin to 0.97V. Connect the feedback resistor divider tap to this pin. Also, connect a phase lead capacitor between FB and VOUT. Typically, this capacitor is 1pF to 10pF.
Corner Pins: These pins are for mechanical support only and can be tied anywhere on the PCB, typically ground.
LT8645S/LT8646S
14Rev. B
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BLOCK DIAGRAM
++–
+–
SLOPE COMP
INTERNAL 0.97V REF
OSCILLATOR200kHz TO 2.2MHz
BURSTDETECT
3.4VREG
M1
M2
CBST0.22µF
COUT
VOUT
8645s BD
SW L
BST
12-16
21-23
ERRORAMP
SHDN
±8%
VC
SHDNTHERMAL SHDNINTVCC UVLOVIN UVLO
SHDNTHERMAL SHDNVIN UVLO
EN/UV1.01V +
–25
11
GND33-38
INTVCC 2
BIAS1
VIN13
GND
GND
8-1017-19
CLKOUT
PG31
FB
R1
27
C1
R3OPT
R4OPT
R2
RT
CSSOPT
VOUT
32
TR/SS1.9µA
60k
INTVCC
29
RT26
SYNC/MODE28
VIN
4-6VIN
COPT1CIN3CIN120nF
CVCC2.2µF
COPT2CIN220nF
SWITCHLOGICANDANTI-
SHOOTTHROUGH
600k
LT8645S ONLY
30LT8645S ONLY
VCLT8646S ONLY30
CF
CC
RC
LT8645S/LT8646S
15Rev. B
For more information www.analog.com
OPERATIONThe LT8645S/LT8646S is a monolithic, constant frequency, current mode step-down DC/DC converter. An oscillator, with frequency set using a resistor on the RT pin, turns on the internal top power switch at the beginning of each clock cycle. Current in the inductor then increases until the top switch current comparator trips and turns off the top power switch. The peak inductor current at which the top switch turns off is controlled by the voltage on the internal VC node. The error amplifier servos the VC node by comparing the voltage on the VFB pin with an internal 0.97V reference. When the load current increases it causes a reduction in the feedback voltage relative to the reference leading the error amplifier to raise the VC voltage until the average inductor current matches the new load current. When the top power switch turns off, the synchronous power switch turns on until the next clock cycle begins or inductor current falls to zero. If overload conditions result in more than 11A flowing through the bottom switch, the next clock cycle will be delayed until switch current returns to a safe level.
The “S” in LT8645S/LT8646S refers to the second genera-tion Silent Switcher technology. This technology allows fast switching edges for high efficiency at high switching frequencies, while simultaneously achieving good EMI performance. This includes the integration of ceramic capacitors into the package for VIN, BST, and INTVCC (see Block Diagram). These caps keep all the fast AC current loops small, which improves EMI performance.
If the EN/UV pin is low, the LT8645S/LT8646S is shut down and draws approximately 1µA from the input. When the EN/UV pin is above 1.01V, the switching regulator will become active.
To optimize efficiency at light loads, the LT8645S/LT8646S operates in Burst Mode operation in light load situations. Between bursts, all circuitry associated with controlling the output switch is shut down, reducing the input sup-ply current to 1.7μA (LT8645S) or 230μA (LT8646S with BIAS = 0). In a typical application, 2.5μA (LT8645S) or 120μA (LT8646S with BIAS = 5VOUT) will be consumed from the input supply when regulating with no load. The SYNC/MODE pin is tied low to use Burst Mode operation
and can be floated to use pulse-skipping mode. If a clock is applied to the SYNC/MODE pin, the part will synchronize to an external clock frequency and operate in pulse-skipping mode. While in pulse-skipping mode the oscillator operates continuously and positive SW transitions are aligned to the clock. During light loads, switch pulses are skipped to regulate the output and the quiescent current will be several hundred µA.
To improve EMI, the LT8645S/LT8646S can operate in spread spectrum mode. This feature varies the clock with a triangular frequency modulation of +20%. For example, if the LT8645S/LT8646S’s frequency is programmed to switch at 2MHz, spread spectrum mode will modulate the oscillator between 2MHz and 2.4MHz. The SYNC/MODE pin should be tied high to INTVCC (~3.4V) or an external supply of 3V to 4V to enable spread spectrum modulation with pulse-skipping mode.
To improve efficiency across all loads, supply current to internal circuitry can be sourced from the BIAS pin when biased at 3.3V or above. Else, the internal circuitry will draw current from VIN. The BIAS pin should be connected to VOUT if the LT8645S/LT8646S output is programmed at 3.3V to 25V.
The VC pin optimizes the loop compensation of the switching regulator based on the programmed switching frequency, allowing for a fast transient response. The VC pin also enables current sharing and a CLKOUT pin enables synchronizing other regulators to the LT8646S.
Comparators monitoring the FB pin voltage will pull the PG pin low if the output voltage varies more than ±8% (typical) from the set point, or if a fault condition is present.
The oscillator reduces the LT8645S/LT8646S’s operat-ing frequency when the voltage at the FB pin is low. This frequency foldback helps to control the inductor current when the output voltage is lower than the programmed value which occurs during start-up or overcurrent condi-tions. When a clock is applied to the SYNC/MODE pin, the SYNC/MODE pin is floated, or held DC high, the frequency foldback is disabled and the switching frequency will slow down only during overcurrent conditions.
LT8645S/LT8646S
16Rev. B
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APPLICATIONS INFORMATIONLow EMI PCB Layout
The LT8645S/LT8646S is specifically designed to mini-mize EMI emissions and also to maximize efficiency when switching at high frequencies. For optimal performance the LT8645S/LT8646S should use multiple VIN bypass capacitors.
Two small 0.47µF capacitors can be placed as close as possible to the LT8645S/LT8646S: One capacitor on each side of the device (COPT1, COPT2). A third capacitor with a larger value, 4.7µF or higher, should be placed near COPT1 or COPT2.
See Figure 1 for recommended PCB layouts.
Figure 1. Recommended PCB Layouts for the LT8645S and LT8646S
8645S F01a
GROUND VIA VIN VIA VOUT VIA OTHER SIGNAL VIASV
V
V
V
V
C1
R2
R1
CSS
RT
COPT1CIN3
COUT
L
a) LT8645S
COPT2
8645S F01b
GROUND VIA VIN VIA VOUT VIA OTHER SIGNAL VIASV
V
VV
C1
R2
R1
CSS
RT
COPT1CIN3
COUT
L
b) LT8646S
COPT2
V
CFRC
CC
LT8645S/LT8646S
17Rev. B
For more information www.analog.com
APPLICATIONS INFORMATIONFor more detail and PCB design files refer to the Demo Board guide for the LT8645S/LT8646S.
Note that large, switched currents flow in the LT8645S/LT8646S VIN and GND pins and the input capacitors. The loops formed by the input capacitors should be as small as possible by placing the capacitors adjacent to the VIN and GND pins. Capacitors with small case size such as 0603 or 0805 are optimal due to lowest parasitic inductance.
The input capacitors, along with the inductor and output capacitors, should be placed on the same side of the circuit board, and their connections should be made on that layer. Place a local, unbroken ground plane under the application circuit on the layer closest to the surface layer. The SW and BOOST nodes should be as small as possible. Finally, keep the FB and RT nodes small so that the ground traces will shield them from the SW and BOOST nodes.
The exposed pads on the bottom of the package should be soldered to the PCB to reduce thermal resistance to ambient. To keep thermal resistance low, extend the ground plane from the GND pins as much as possible, and add thermal vias to additional ground planes within the circuit board and on the bottom side.
Burst Mode Operation
To enhance efficiency at light loads, the LT8645S/LT8646S operates in low ripple Burst Mode operation, which keeps the output capacitor charged to the desired output voltage while minimizing the input quiescent current and minimiz-ing output voltage ripple. In Burst Mode operation the LT8645S/LT8646S delivers single small pulses of current to the output capacitor followed by sleep periods where the output power is supplied by the output capacitor. While in sleep mode the LT8645S consumes 1.7μA. and the LT8646S consumes 230μA.
As the output load decreases, the frequency of single cur-rent pulses decreases (see Figure 2a) and the percentage of time the LT8645S/LT8646S is in sleep mode increases, resulting in much higher light load efficiency than for typi-cal converters. By maximizing the time between pulses, the LT8645S’s quiescent current approaches 2.5µA for a typical application when there is no output load. Therefore, to optimize the quiescent current performance at light
Figure 2. SW Frequency vs Load Information in Burst Mode Operation (2a) and Pulse-Skipping Mode (2b)
(2a) (2b)
Burst Frequency
FRONT PAGE APPLICATIONVIN = 12VVOUT = 5V
LOAD CURRENT (mA)0 100 200 300 400 500 600
0
200
400
600
800
1000
1200
SWIT
CHIN
G FR
EQUE
NCY
(kHz
)
8645S F02a
Minimum Load to Full Frequency (Pulse-Skipping Mode)
FRONT PAGE APPLICATIONVOUT = 5VfSW = 1MHzVSYNC = Float
INPUT VOLTAGE (V)5 15 25 35 45 55 65
0
50
100
150
200
250
300
350
400
LOAD
CUR
RENT
(mA)
8645S F02b
LT8645S/LT8646S
18Rev. B
For more information www.analog.com
APPLICATIONS INFORMATIONloads, the current in the feedback resistor divider must be minimized as it appears to the output as load current.
In order to achieve higher light load efficiency, more energy must be delivered to the output during the single small pulses in Burst Mode operation such that the LT8645S/LT8646S can stay in sleep mode longer between each pulse. This can be achieved by using a larger value inductor (i.e., 4.7µH), and should be considered independent of switch-ing frequency when choosing an inductor. For example, while a lower inductor value would typically be used for a high switching frequency application, if high light load efficiency is desired, a higher inductor value should be chosen. See curve in Typical Performance Characteristics.
While in Burst Mode operation the current limit of the top switch is approximately 1.25A (as shown in Figure 3), resulting in low output voltage ripple. Increasing the output capacitance will decrease output ripple proportionally. As load ramps upward from zero the switching frequency
will increase but only up to the switching frequency pro-grammed by the resistor at the RT pin as shown in Figure 2a.
The output load at which the LT8645S/LT8646S reaches the programmed frequency varies based on input voltage, output voltage, and inductor choice. To select low ripple Burst Mode operation, tie the SYNC/MODE pin below 0.4V (this can be ground or a logic low output).
Pulse-Skipping Mode
For some applications it is desirable for the LT8645S/LT8646S to operate in pulse-skipping mode, offering two major differences from Burst Mode operation. First is the clock stays awake at all times and all switching cycles are aligned to the clock. In this mode much of the internal circuitry is awake at all times, increasing quiescent cur-rent to several hundred µA. Second is that full switching frequency is reached at lower output load than in Burst Mode operation (see Figure 2b). To enable pulse-skipping mode, float the SYNC/MODE pin. Leakage current in this pin should be <1µA. See Block Diagram for internal pull-up and pull-down resistance.
Spread Spectrum Mode
The LT8645S/LT8646S features spread spectrum opera-tion to further reduce EMI emissions. To enable spread spectrum operation, the SYNC/MODE pin should be tied high to INTVCC (~3.4V) or an external supply of 3V to 4V. In this mode, triangular frequency modulation is used to vary the switching frequency between the value pro-grammed by RT to approximately 20% higher than that value. The modulation frequency is approximately 3kHz. For example, when the LT8645S/LT8646S is programmed to 2MHz, the frequency will vary from 2MHz to 2.4MHz at
Figure 3. Burst Mode Operation
Switching Waveforms, Burst Mode Operation
FRONT PAGE APPLICATION12VIN TO 5VOUT AT 10mAVSYNC = 0V
10µs/DIV
VSW5V/DIV
IL500mA/DIV
8645S F03
LT8645S/LT8646S
19Rev. B
For more information www.analog.com
APPLICATIONS INFORMATIONa 3kHz rate. When spread spectrum operation is selected, Burst Mode operation is disabled, and the part will run in pulse-skipping mode.
Synchronization
To synchronize the LT8645S/LT8646S oscillator to an external frequency, connect a square wave to the SYNC/MODE pin. The square wave amplitude should have valleys that are below 0.4V and peaks above 1.5V (up to 6V), with a minimum on-time and off-time of 50ns.
The LT8645S/LT8646S will not enter Burst Mode opera-tion at low output loads while synchronized to an external clock, but instead will pulse-skip to maintain regulation. The LT8645S/LT8646S may be synchronized over a 200kHz to 2.2MHz range. The RT resistor should be chosen to set the LT8645S/LT8646S switching frequency equal to or below the lowest synchronization input. For example, if the synchronization signal will be 500kHz and higher, the RT should be selected for 500kHz. The slope compensation is set by the RT value, while the minimum slope compensation required to avoid subharmonic oscillations is established by the inductor size, input voltage, and output voltage. Since the synchronization frequency will not change the slopes of the inductor current waveform, if the inductor is large enough to avoid subharmonic oscillations at the frequency set by RT, then the slope compensation will be sufficient for all synchronization frequencies.
The LT8645S/LT8646S does not operate in forced continu-ous mode regardless of SYNC/MODE signal.
FB Resistor Network
The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the resistor values according to:
R1=R2
VOUT0.97V
−1⎛⎝⎜
⎞⎠⎟
(1)
Reference designators refer to the Block Diagram. 1% resistors are recommended to maintain output voltage accuracy.
For the LT8645S, if low input quiescent current and good light-load efficiency are desired, use large resistor values for the FB resistor divider. The current flowing in the divider acts as a load current, and will increase the no-load input current to the converter, which is approximately:
IQ = 1.7µA +
VOUTR1+R2
⎛⎝⎜
⎞⎠⎟
VOUTVIN
⎛⎝⎜
⎞⎠⎟
1n
⎛⎝⎜
⎞⎠⎟
(2)
where 1.7µA is the quiescent current of the LT8645S and the second term is the current in the feedback divider reflected to the input of the buck operating at its light load efficiency n. For a 3.3V application with R1 = 1M and R2 = 412k, the feedback divider draws 2.3µA. With VIN = 12V and n = 80%, this adds 0.8µA to the 1.7µA quiescent current resulting in 2.5µA no-load current from the 12V supply. Note that this equation implies that the no-load current is a function of VIN; this is plotted in the Typical Performance Characteristics section.
When using large FB resistors, a 1pF to 10pF phase-lead capacitor should be connected from VOUT to FB.
Setting the Switching Frequency
The LT8645S/LT8646S uses a constant frequency PWM architecture that can be programmed to switch from 200kHz to 2.2MHz by using a resistor tied from the RT pin to ground. A table showing the necessary RT value for a desired switching frequency is in Table 1.
The RT resistor required for a desired switching frequency can be calculated using:
RT =
46.5fSW
– 5.2
(3)
where RT is in kΩ and fSW is the desired switching fre-quency in MHz.
LT8645S/LT8646S
20Rev. B
For more information www.analog.com
APPLICATIONS INFORMATIONTable 1. SW Frequency vs RT Value
fSW (MHz) RT (kΩ)
0.2 232
0.3 150
0.4 110
0.5 88.7
0.6 71.5
0.7 60.4
0.8 52.3
1.0 41.2
1.2 33.2
1.4 28.0
1.6 23.7
1.8 20.5
2.0 17.8
2.2 15.8
Operating Frequency Selection and Trade-Offs
Selection of the operating frequency is a trade-off between efficiency, component size, and input voltage range. The advantage of high frequency operation is that smaller induc-tor and capacitor values may be used. The disadvantages are lower efficiency and a smaller input voltage range.
The highest switching frequency (fSW(MAX)) for a given application can be calculated as follows:
fSW(MAX) =VOUT + VSW(BOT)
tON(MIN) VIN – VSW(TOP) + VSW(BOT)( )
(4)
where VIN is the typical input voltage, VOUT is the output voltage, VSW(TOP) and VSW(BOT) are the internal switch drops (~0.3V, ~0.2V, respectively at maximum load) and tON(MIN) is the minimum top switch on-time (see the Elec-trical Characteristics). This equation shows that a slower switching frequency is necessary to accommodate a high VIN/VOUT ratio.
For transient operation, VIN may go as high as the abso-lute maximum rating of 65V regardless of the RT value, however the LT8645S/LT8646S will reduce switching frequency as necessary to maintain control of inductor current to assure safe operation.
The LT8645S/LT8646S is capable of a maximum duty cycle of approximately 99%, and the VIN-to-VOUT dropout is limited by the RDS(ON) of the top switch. In this mode the LT8645S/LT8646S skips switch cycles, resulting in a lower switching frequency than programmed by RT.
For applications that cannot allow deviation from the pro-grammed switching frequency at low VIN/VOUT ratios use the following formula to set switching frequency:
VIN(MIN) =
VOUT + VSW(BOT)
1– fSW • tOFF(MIN)– VSW(BOT) + VSW(TOP)
(5)
where VIN(MIN) is the minimum input voltage without skipped cycles, VOUT is the output voltage, VSW(TOP) and VSW(BOT) are the internal switch drops (~0.3V, ~0.2V, respectively at maximum load), fSW is the switching fre-quency (set by RT), and tOFF(MIN) is the minimum switch off-time. Note that higher switching frequency will increase the minimum input voltage below which cycles will be dropped to achieve higher duty cycle.
Inductor Selection and Maximum Output Current
The LT8645S/LT8646S is designed to minimize solution size by allowing the inductor to be chosen based on the output load requirements of the application. During over-load or short-circuit conditions the LT8645S/LT8646S safely tolerates operation with a saturated inductor through the use of a high speed peak-current mode architecture.
A good first choice for the inductor value is:
L =
VOUT + VSW(BOT)
fSW
⎛
⎝⎜
⎞
⎠⎟ • 0.4
(6)
where fSW is the switching frequency in MHz, VOUT is the output voltage, VSW(BOT) is the bottom switch drop (~0.2V) and L is the inductor value in μH.
To avoid overheating and poor efficiency, an inductor must be chosen with an RMS current rating that is greater than the maximum expected output load of the application. In addition, the saturation current (typically labeled ISAT)
LT8645S/LT8646S
21Rev. B
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APPLICATIONS INFORMATIONrating of the inductor must be higher than the load current plus 1/2 of in inductor ripple current:
IL(PEAK) = ILOAD(MAX) +
12ΔIL
(7)
where ∆IL is the inductor ripple current as calculated in Equation 9 and ILOAD(MAX) is the maximum output load for a given application.
As a quick example, an application requiring 2A output should use an inductor with an RMS rating of greater than 2A and an ISAT of greater than 3A. During long duration overload or short-circuit conditions, the inductor RMS rating requirement is greater to avoid overheating of the inductor. To keep the efficiency high, the series resistance (DCR) should be less than 0.02Ω, and the core material should be intended for high frequency applications.
The LT8645S/LT8646S limits the peak switch current in order to protect the switches and the system from overload faults. The top switch current limit (ILIM) is 14A at low duty cycles and decreases linearly to 11.5A at DC = 0.9. The inductor value must then be sufficient to supply the desired maximum output current (IOUT(MAX)), which is a function of the switch current limit (ILIM) and the ripple current.
IOUT(MAX) = ILIM –
ΔIL2
(8)
The peak-to-peak ripple current in the inductor can be calculated as follows:
ΔIL =
VOUTL • fSW
• 1–VOUT
VIN(MAX)
⎛
⎝⎜
⎞
⎠⎟
(9)
where fSW is the switching frequency of the LT8645S/LT8646S, and L is the value of the inductor. Therefore, the maximum output current that the LT8645S/LT8646S will deliver depends on the switch current limit, the inductor value, and the input and output voltages. The inductor value may have to be increased if the inductor ripple cur-rent does not allow sufficient maximum output current (IOUT(MAX)) given the switching frequency, and maximum input voltage used in the desired application.
When operating at high VIN (greater than 40V) and at a frequency and duty cycle that would require a switch on-
time of less than 100ns, choose an inductor such that the ∆IL is greater than 1.5A in order to prevent duty cycle jitter.
In order to achieve higher light load efficiency, more energy must be delivered to the output during the single small pulses in Burst Mode operation such that the LT8645S/LT8646S can stay in sleep mode longer between each pulse. This can be achieved by using a larger value inductor (i.e., 4.7µH), and should be considered independent of switch-ing frequency when choosing an inductor. For example, while a lower inductor value would typically be used for a high switching frequency application, if high light load efficiency is desired, a higher inductor value should be chosen. See curve in Typical Performance Characteristics.
The optimum inductor for a given application may differ from the one indicated by this design guide. A larger value inductor provides a higher maximum load current and reduces the output voltage ripple. For applications requiring smaller load currents, the value of the inductor may be lower and the LT8645S/LT8646S may operate with higher ripple current. This allows use of a physically smaller inductor, or one with a lower DCR resulting in higher efficiency. Be aware that low inductance may result in discontinuous mode operation, which further reduces maximum load current.
For more information about maximum output current and discontinuous operation, see Analog Devices’ Application Note 44.
For duty cycles greater than 50% (VOUT/VIN > 0.5), a minimum inductance is required to avoid subharmonic oscillation (See Equation 10). See Application Note 19 for more details.
LMIN =
VIN(2 •DC – 1)3 • fSW
(10)
where DC is the duty cycle ratio (VOUT/VIN) and fSW is the switching frequency.
Input Capacitors
The VIN of the LT8645S/LT8646S should be bypassed with at least three ceramic capacitors for best perfor-mance. Two small ceramic capacitors of 0.47µF can be placed close to the part; one on each side of the device
LT8645S/LT8646S
22Rev. B
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APPLICATIONS INFORMATION(COPT1, COPT2). These capacitors should be 0603 or 0805 in size. For automotive applications requiring 2 series input capacitors, two small 0603 or 0805 may be placed at each side of the LT8645S/LT8646S.
A third, larger ceramic capacitor of 4.7µF or larger should be placed close to COPT1 or COPT2. See layout section for more detail. X7R or X5R capacitors are recommended for best performance across temperature and input voltage variations.
Note that larger input capacitance is required when a lower switching frequency is used. If the input power source has high impedance, or there is significant inductance due to long wires or cables, additional bulk capacitance may be necessary. This can be provided with a low performance electrolytic capacitor.
A ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the LT8645S/LT8646S circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT8645S/LT8646S’s voltage rating. This situation is easily avoided (see Analog Devices Application Note 88).
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along with the inductor, it filters the square wave generated by the LT8645S/LT8646S to produce the DC output. In this role it determines the output ripple, thus low impedance at the switching frequency is important. The second function is to store energy in order to satisfy transient loads and stabilize the LT8645S/LT8646S’s control loop. Ceramic capacitors have very low equivalent series resistance (ESR) and provide the best ripple performance. For good starting values, see the Typical Applications section.
Use X5R or X7R types. This choice will provide low output ripple and good transient response. Transient performance can be improved with a higher value output capacitor and the addition of a feedforward capacitor placed between VOUT and FB. Increasing the output capacitance will also decrease the output voltage ripple. A lower value of output capacitor can be used to save space and cost but transient performance will suffer and may cause loop instability. See
the Typical Applications in this data sheet for suggested capacitor values.
When choosing a capacitor, special attention should be given to the data sheet to calculate the effective capacitance under the relevant operating conditions of voltage bias and temperature. A physically larger capacitor or one with a higher voltage rating may be required.
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low ESR. However, ceramic capacitors can cause problems when used with the LT8645S/LT8646S due to their piezoelectric nature. When in Burst Mode operation, the LT8645S/LT8646S’s switching frequency depends on the load current, and at very light loads the LT8645S/LT8646S can excite the ceramic capacitor at audio frequencies, generating audible noise. Since the LT8645S/LT8646S operates at a lower current limit during Burst Mode op-eration, the noise is typically very quiet to a casual ear. If this is unacceptable, use a high performance tantalum or electrolytic capacitor at the output. Low noise ceramic capacitors are also available.
A final precaution regarding ceramic capacitors concerns the maximum input voltage rating of the LT8645S/LT8646S. As previously mentioned, a ceramic input capacitor combined with trace or cable inductance forms a high quality (un-derdamped) tank circuit. If the LT8645S/LT8646S circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT8645S/LT8646S’s rating. This situation is easily avoided (see Analog Devices Application Note 88).
Enable Pin
The LT8645S/LT8646S is in shutdown when the EN pin is low and active when the pin is high. The rising threshold of the EN comparator is 1.01V, with 45mV of hysteresis. The EN pin can be tied to VIN if the shutdown feature is not used, or tied to a logic level if shutdown control is required.
Adding a resistor divider from VIN to EN programs the LT8645S/LT8646S to regulate the output only when VIN is above a desired voltage (see the Block Diagram). Typically, this threshold, VIN(EN), is used in situations where the input
LT8645S/LT8646S
23Rev. B
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APPLICATIONS INFORMATIONsupply is current limited, or has a relatively high source resistance. A switching regulator draws constant power from the source, so source current increases as source voltage drops. This looks like a negative resistance load to the source and can cause the source to current limit or latch low under low source voltage conditions. The VIN(EN) threshold prevents the regulator from operating at source voltages where the problems might occur. This threshold can be adjusted by setting the values R3 and R4 such that they satisfy the following equation:
VIN(EN) =
R3R4
+1⎛⎝⎜
⎞⎠⎟ • 1.01V
(11)
where the LT8645S/LT8646S will remain off until VIN is above VIN(EN). Due to the comparator’s hysteresis, switch-ing will not stop until the input falls slightly below VIN(EN).
When operating in Burst Mode operation for light load currents, the current through the VIN(EN) resistor network can easily be greater than the supply current consumed by the LT8645S/LT8646S. Therefore, the VIN(EN) resistors should be large to minimize their effect on efficiency at low loads.
INTVCC Regulator
An internal low dropout (LDO) regulator produces the 3.4V supply from VIN that powers the drivers and the internal bias circuitry. The INTVCC can supply enough current for the LT8645S/LT8646S’s circuitry. To improve efficiency the internal LDO can also draw current from the BIAS pin when the BIAS pin is at 3.1V or higher. Typically the BIAS pin can be tied to the output of the LT8645S/LT8646S, or can be tied to an external supply of 3.3V or above. If BIAS is connected to a supply other than VOUT, be sure to bypass with a local ceramic capacitor. If the BIAS pin is below 3.0V, the internal LDO will consume current from VIN. Applications with high input voltage and high switching frequency where the internal LDO pulls current from VIN will increase die temperature because of the higher power dissipation across the LDO. Do not connect an external load to the INTVCC pin.
Frequency Compensation (LT8646S Only)
Loop compensation determines the stability and transient performance, and is provided by the components tied to the VC pin. Generally, a capacitor (CC) and a resistor (RC) in series to ground are used. Designing the compensation network is a bit complicated and the best values depend on the application. A practical approach is to start with one of the circuits in this data sheet that is similar to your application and tune the compensation network to opti-mize the performance. LTspice® simulations can help in this process. Stability should then be checked across all operating conditions, including load current, input voltage and temperature. The LT1375 data sheet contains a more thorough discussion of loop compensation and describes how to test the stability using a transient load.
Figure 4 shows an equivalent circuit for the LT8646S control loop. The error amplifier is a transconductance amplifier with finite output impedance. The power section, consisting of the modulator, power switches, and inductor, is modeled as a transconductance amplifier generating an output current proportional to the voltage at the VC pin. Note that the output capacitor integrates this current, and that the capacitor on the VC pin (CC) integrates the error amplifier output current, resulting in two poles in the loop. A zero is required and comes from a resistor RC in series with CC. This simple model works well as long as the value
VC
200k
FB
0.97V
8645S F04
CPL
LT8646S
CURRENT MODEPOWER STAGE
+–
COUT
R1
R2
OUTPUT
CFCC
RC
Gm = 8S
gm = 1.7mS
Figure 4. Model for Loop Response
LT8645S/LT8646S
24Rev. B
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APPLICATIONS INFORMATIONof the inductor is not too high and the loop crossover frequency is much lower than the switching frequency. A phase lead capacitor (Cpl) across the feedback divider can be used to improve the transient response and is required to cancel the parasitic pole caused by the feedback node to ground capacitance.
Output Voltage Tracking and Soft-Start
The LT8645S/LT8646S allows the user to program its out-put voltage ramp rate by means of the TR/SS pin. An internal 1.9μA pulls up the TR/SS pin to INTVCC. Putting an external capacitor on TR/SS enables soft starting the output to prevent current surge on the input supply. During the soft-start ramp the output voltage will proportionally track the TR/SS pin voltage.
For output tracking applications, TR/SS can be externally driven by another voltage source. For the LT8645S, from 0V to 0.97V, the TR/SS voltage will override the internal 0.97V reference input to the error amplifier, thus regulating the FB pin voltage to that of TR/SS pin. When TR/SS is above 0.97V, tracking is disabled and the feedback volt-age will regulate to the internal reference voltage. For the LT8646S, from 0V to 1.6V, the TR/SS voltage will override the internal 0.97V reference input to the error amplifier, thus regulating the FB pin voltage to a function of the TR/SS pin. See plot in the Typical Performance Characteristics section. When TR/SS is above 1.6V, tracking is disabled and the feedback voltage will regulate to the internal ref-erence voltage. The TR/SS pin may be left floating if the function is not needed.
An active pull-down circuit is connected to the TR/SS pin which will discharge the external soft-start capacitor in the case of fault conditions and restart the ramp when the faults are cleared. Fault conditions that clear the soft-start capacitor are the EN/UV pin transitioning low, VIN voltage falling too low, or thermal shutdown.
Output Power Good
When the LT8645S/LT8646S’s output voltage is within the ±8% window of the regulation point, the output voltage is considered good and the open-drain PG pin goes high impedance and is typically pulled high with an external resistor. Otherwise, the internal pull-down device will pull
the PG pin low. To prevent glitching both the upper and lower thresholds include 0.4% of hysteresis. PG is valid when VIN is above 3.4V
The PG pin is also actively pulled low during several fault conditions: EN/UV pin is below 1V, INTVCC has fallen too low, VIN is too low, or thermal shutdown.
Paralleling (LT8646S Only)
To increase the possible output current, two LT8646Ss can be connected in parallel to the same output. To do this, the VC and FB pins are connected together, and each LT8646S’s SW node is connected to the common output through its own inductor. The CLKOUT pin of one LT8646S should be connected to the SYNC/MODE pin of the second LT8646S to have both devices operate in the same mode. During pulse-skipping, Spread Spectrum, and Synchronization modes, both devices will operate at the same frequency. Figure 5 shows an application where two LT8646S are paralleled to get one output capable of up to 16A.
Figure 5. Paralleling Two LT8646S
LT8646S
8645S F05
SW
CLKOUT
VC
FB
LT8646S
SS
SS
FB
VC
SYNC/MODE
SW
COUTR1 C1
VOUT16A
L1
L2
R2RC
CC
Shorted and Reversed Input Protection
The LT8645S/LT8646S will tolerate a shorted output. Several features are used for protection during output short-circuit and brownout conditions. The first is the switching frequency will be folded back while the output is lower than the set point to maintain inductor current control. Second, the bottom switch current is monitored such that if inductor current is beyond safe levels switch-ing of the top switch will be delayed until such time as the inductor current falls to safe levels.
LT8645S/LT8646S
25Rev. B
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APPLICATIONS INFORMATION
Figure 7. Case Temperature Rise
Frequency foldback behavior depends on the state of the SYNC pin: If the SYNC pin is low the switching frequency will slow while the output voltage is lower than the pro-grammed level. If the SYNC pin is connected to a clock source, floated, or tied high, the LT8645S/LT8646S will stay at the programmed frequency without foldback and only slow switching if the inductor current exceeds safe levels.
There is another situation to consider in systems where the output will be held high when the input to the LT8645S/LT8646S is absent. This may occur in battery charg-ing applications or in battery-backup systems where a battery or some other supply is diode ORed with the LT8645S/LT8646S’s output. If the VIN pin is allowed to float and the EN pin is held high (either by a logic signal or because it is tied to VIN), then the LT8645S/LT8646S’s internal circuitry will pull its quiescent current through its SW pin. This is acceptable if the system can tolerate several μA in this state. If the EN pin is grounded the SW pin current will drop to near 1µA. However, if the VIN pin is grounded while the output is held high, regardless of EN, parasitic body diodes inside the LT8645S/LT8646S can pull current from the output through the SW pin and the VIN pin. Figure 6 shows a connection of the VIN and EN/UV pins that will allow the LT8645S/LT8646S to run only when the input voltage is present and that protects against a shorted or reversed input.
the LT8645S/LT8646S. Placing additional vias can reduce thermal resistance further. The maximum load current should be derated as the ambient temperature approaches the maximum junction rating. Power dissipation within the LT8645S/LT8646S can be estimated by calculating the total power loss from an efficiency measurement and subtract-ing the inductor loss. The die temperature is calculated by multiplying the LT8645S/LT8646S power dissipation by the thermal resistance from junction to ambient.
The internal overtemperature protection monitors the junc-tion temperature of the LT8645S/LT8646S. If the junction temperature reaches approximately 180°C, the LT8645S/LT8646S will stop switching and indicate a fault condition until the temperature drops about 10°C cooler.
Temperature rise of the LT8645S/LT8646S is worst when operating at high load, high VIN, and high switching fre-quency. If the case temperature is too high for a given application, then either VIN, switching frequency, or load current can be decreased to reduce the temperature to an acceptable level. Figure 7 shows examples of how case temperature rise can be managed by reducing VIN, switching frequency, or load.
Figure 6. Reverse VIN Protection
VINVIN
D1
LT8645S/LT8646S
EN/UV
8645S F06
GND
DC2468A DEMO BOARDVIN = 12V, fSW = 500kHzVIN = 24V, fSW = 500kHzVIN = 12V, fSW = 2MHzVIN = 24V, fSW = 2MHz
LOAD CURRENT (A)0 1 2 3 4 5 6 7 8
0
10
20
30
40
50
60
70
80
CASE
TEM
PERA
TURE
RIS
E (°
C)
8645S F07
Thermal Considerations
For higher ambient temperatures, care should be taken in the layout of the PCB to ensure good heat sinking of the LT8645S/LT8646S. The ground pins on the bottom of the package should be soldered to a ground plane. This ground should be tied to large copper layers below with thermal vias; these layers will spread heat dissipated by
The LT8645S/LT8646S’s top switch current limit decreases with higher duty cycle operation for slope compensation. This also limits the output current the LT8645S/LT8646S can deliver for a given application. See curve in the Typical Performance Characteristics section.
LT8645S/LT8646S
26Rev. B
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Figure 8. 5V 8A Step-Down Converter with Soft-Start and Power Good
Figure 10. Ultralow EMI 5V, 8A Step-Down Converter with Spread Spectrum
TYPICAL APPLICATIONS
Figure 9. 3.3V, 8A Step-Down Converter with Soft Start and Power Good
LT8645S/LT8646S
PG
BIAS
FB
VIN3.8V TO 65V
GND
RT
VINEN/UV
VOUT3.3V8A
2.2pF
4.7µF
10nF
1M
412k
f SW = 500kHzL: LHMI-8040
88.7k
TR/SS
SYNC/MODECLKOUT
SW
100k
3.3µH
8645S F09
47µF×21210X5R/X7R
VC*6.49k
PINS NOT USED IN THIS CIRCUIT:BST, INTVCC
*VC PIN AND COMPONENTSONLY APPLY TO LT8646S.
1.5nF
LT8645S/LT8646S
VIN
SWBIAS
FB
VIN5.5V TO 65V
GND
PINS NOT USED IN THIS CIRCUIT:BST, CLKOUT, PG,TR/SS
*VC PIN AND COMPONENTSONLY APPLY TO LT8646S.
RT
VINEN/UV
VOUT5V8A
100µF1210X5R/X7R
4.7pF
0.47µF0805
0.47µF0805
4.7µF1210
17.8k
1M
243k
8645S F10
1µH
fSW = 2MHzL: XEL6030FB1 BEAD: WE-MPSB 100Ω 8A 1812
GND GND
FB1BEAD
2.2µF1210
2.2µF1210
INTVCC
SYNC/MODE
VC*7.5k
330pF
LT8645S/LT8646S
PGBIAS
FB
VIN5.5V TO 65V
GND
PINS NOT USED IN THIS CIRCUIT:BST, INTVCC
*VC PIN AND COMPONENTSONLY APPLY TO LT8646S.
RT
VINEN/UV
VOUT5V8A
47µF×21210X5R/X7R
2.2pF
4.7µF
88.7k
1M
243k
fSW = 500kHzL: LHMI-8040
10nF
TR/SS
VC*SYNC/MODECLKOUT
SW
100k
3.3µH
8645S F08
6.04k
1nF
LT8645S/LT8646S
27Rev. B
For more information www.analog.com
Figure 11. 2MHz 5V, 8A Step-Down Converter
Figure 13. 12V, 8A Step-Down Converter
TYPICAL APPLICATIONS
Figure 12. 2MHz 3.3V, 8A Step-Down Converter
LT8645S/LT8646S
SW
BIAS
FB
VIN5.5V TO 65V
GND
PINS NOT USED IN THIS CIRCUIT:BST, CLKOUT, INTVCC, PG, SYNC/MODE, TR/SS
*VC PIN AND COMPONENTSONLY APPLY TO LT8646S.
RT
VIN
EN/UV
VOUT5V8A
100µF1210X5R/X7R
4.7pF
8645S F11
4.7µF
17.8k
1M
243k
1µH
fSW = 2MHzL: XEL6030
VC*7.5k
330pF
LT8645S/LT8646S
SW
BIAS
FB
VIN3.8V TO 42V
(65V TRANSIENT)
GND
PINS NOT USED IN THIS CIRCUIT:BST, CLKOUT, INTVCC, PG, SYNC/MODE, TR/SS
*VC PIN AND COMPONENTSONLY APPLY TO LT8646S.
RT
VIN
EN/UV
VOUT3.3V8A
100µF1210X5R/X7R
4.7pF
8645S F12
4.7µF
17.8k
1M
412k
0.82µH
fSW = 2MHzL: XEL6030
VC*8.87k
330pF
LT8645S
SW
BIAS
FB
VIN12.5V TO 65V
GND
RT
VIN
EN/UV
VOUT12V8A
47µF1210X5R/X7R
2.2pF
8645S F13
4.7µF
41.2k
1M
88.7k
4.7µH
fSW = 1MHzL: XEL6060
PINS NOT USED IN THIS CIRCUIT:BST, CLKOUT, INTVCC, PG, SYNC/MODE, TR/SS
LT8645S/LT8646S
28Rev. B
For more information www.analog.com
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AGE
OUTL
INE
0.25
±0.
05
0.70
±0.
05
6.50
±0.
05
4.50
±0.
05
L
e/2
0.37
5
0.37
5
MIN
0.85
0.01
0.30
0.22
LT8645S/LT8646S
29Rev. B
For more information www.analog.com
Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
REVISION HISTORYREV DATE DESCRIPTION PAGE NUMBER
A 01/18 Added LT8646S All
B 04/20 Added AEC-Q100 qualified statement.Inserted devices options table.Removed EMC word.Updated Pin Configuration figure descriptions.Updated Ordering Information table with #W devices.Text and Figures 4, 5,12 and editing.Update Package Description to match Rev C.
11
All23
21, 22, 2728
LT8645S/LT8646S
30Rev. B
For more information www.analog.com ANALOG DEVICES, INC. 2017-2020
04/20www.analog.com
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PART NUMBER DESCRIPTION COMMENTS
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LT8616 42V, Dual 2.5A + 1.5A, 95% Efficiency, 2.2MHz Synchronous MicroPower Step-Down DC/DC Converter with IQ = 5µA
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LT8645S
SW
BIAS
FB
VIN3.4V TO 30V
(65V TRANSIENT)
GND
RT
VIN
EN/UV
VOUT1.8V8A
4.7pF
4.7µF
41.2k
866k
1M
0.82µH
fSW = 1MHzL: XEL6030
1µF
EXTERNALSOURCE >3.1VOR GND
8645S TA02
47µF×21210X5R/X7R
PINS NOT USED IN THIS CIRCUIT:BST, CLKOUT, INTVCC, PG, SYNC/MODE, TR/SS